In physical vapour deposition (PVD) a metal is vaporized and deposited on a substrate, such as a steel strip. In this process a liquid metal or a liquid of two or more metals is supplied to an evaporator in a vacuum chamber where it is vaporized. In PVD processes and more in particular in continuous or semi-continuous PVD processes on an industrial scale it is necessary to regularly supply liquid metal to the evaporator device to make up for the liquid metal evaporated from the evaporator.
For the supply of liquid metal to the evaporator a supply system is used comprising a feeding vessel with liquid metal, heated pipes between the vessel and evaporator, one or more valves to shut off and open the flow in the pipes and depending on the system one or more pumps to pump the liquid metal. The vessel with liquid metal is located outside the vacuum chamber.
In order to safely start and shut down the PVD installation it is essential that a valve is present in the piping between feeding vessel and the evaporator melt. This will assure that when the piping is empty the vacuum chamber will not be vented. Furthermore, during the shut down procedure or during an emergency situation the valve will make it possible to shut off the supply of liquid metal in a controlled and safe manner.
Liquid metal is used to provide an anti-corrosive coating on a steel strip and usually contains Zn or Zn and Mg which metals are both very reactive in the liquid phase. The material of the valve that comes into contact with the liquid ZnMg should not only be capable to withstand the corrosive action of this liquid metal but it should also be dense enough to prevent penetration of the liquid metal into the material, since such penetration could sooner or later lead to failure of the material.
Furthermore, it is not enough that the material of the valve is resistant against ZnMg. Oxides present on the material, formed by the material and any excess oxygen must be stable enough to liquid ZnMg to assure that the valve will continue to work.